The poster for The Devil's Double, a thriller about Saddam Hussein's son Uday, is a vision of excess in Babylon. Dominic Cooper slouches on a gilded throne looking every inch the playboy, all chest hair and dark shades, two machine guns spewing cartridges over his head.

"Sex ... power ... too much money ... what do you get a 'prince' that has everything?" reads the advertising spiel. But any suggestion that his film, which opens on Wednesday, could glamorise the dying days of Saddam's regime horrifies the director, Lee Tamahori. "I never intended to become an apologist for Tony Blair's war," he wails down the phone from his home in Wellington, New Zealand. "I just wanted to show that Uday was a psycho."

He says he's been waiting all his life to make a cocaine-fuelled, machete-waving romp through the heady days of Saddam's Baghdad. Now he wonders if he hasn't inadvertently justified the war in Iraq, which put an end to all that. "In an odd way, I think a whole bunch of neocons will probably use my damn film as a justification for their invasion."

Tamahori's view is that Blair "lied through his teeth", and he is furious to see the former prime minister giving speeches in New Zealand. Whatever his worries about the film being used for political capital, the Husseins and their psychopathic habits were obviously crying out to be turned into Hollywood capital.

His film depicts 1980s Baghdad as a city of sin, run for the debauched entertainment of a seedy elite. At its centre is Uday, the elder son of the ruling dictator, who was assassinated by US special forces in 2003. He is played by Cooper (who made his name in The History Boys) as a vainglorious, self-obsessed monster. Cooper also plays Latif Yahia, the man supposedly hired as a body double for Uday.

The film has been well received in the US, although there is concern over Yahia's involvement. Yahia is still alive, and was interviewed by Tamahori for the script. Former members of the Baath party have since shed doubt on the credibility of his claims, and there is growing unease that Tamahori has inadvertently done Yahia a favour by taking his story at face value. It is an allegation that Tamahori denies.

"I told Latif this is going to be a fictional version of his life story. I told him I'm not here to clean up your image, because I'm not sure how much of it I can truly believe.

Tamahori's research raises eyebrows for other reasons. He has never, for instance, set foot in Baghdad. And shouldn't the film have been shot in the Middle East, rather than Malta? Conceding that he did "basic research" before "giving up" on it, his main defence is one of style.

"I strived to make this a gangster film, not one that would get lost in the art houses. The more I researched it, the more I realised I was in danger of being hung by the truth. Then every journalist could pull the film apart and historians would tear it to pieces."

The main point of the film, he says, is to entertain. "All film is ultimately entertainment, regardless whether it's The Battle of Algiers or a documentary about penguins," he says. "It is all there to manipulate us. I don't want people to leave thinking they have the truth. What they have instead is an illusion of the truth."

The result is a historically dubious, albeit absorbing, thriller, served up with the style and finesse of Die Another Day, the Bond movie Tamahori directed in 2002 and which propelled him to international stature.

Born in Wellington in June 1950 to a Maori father and a mother of British descent, Tamahori came to prominence in 1994 with the drama Once Were Warriors. Subsequent films include Mulholland Falls.

Today, he remains exercised about what the future holds for Iraq. "It's not hard to see an Iraq that will be rotten to the core and rampant with corruption in a few years. Or a world crawling with mercenaries and bandit capitalists, like you see in Russia."

The conditions are ripe, he says, for another Saddam figure to emerge. "When the US does finally withdraw, it will create a vacuum that will ultimately pave the way for another dictator to capitalise on what is left behind. Perhaps not as strong as Saddam – but a strong, male leader more likely to implement forceful discipline than settle for democracy."

Tamahori appears relatively sympathetic towards Saddam: in the film he appears desperate to maintain discipline in a country on the brink of foreign invasion, with a son whom he wishes had never been born. "We have left Saddam Hussein as a Don Corleone, or a mafia don trying to keep things in check." he says. "It was Saddam who built the country universities, road and free education. That seemed to be the devil's pact that he signed with the people."

Whatever historians make of it, Tamahori's film is one of the most gripping attempts to address the first Gulf War, helped by a stand-out performance from Cooper, who convincingly weaves between the two main roles of Uday and Yahia.

Tamahori may have reason to believe he has gifted the State Department a slice of prime propaganda. But his film is limited to the events before the first Gulf War, and makes no reference to the war of 2003. We can look forward to that in the sequel. The script practically writes itself.

Curriculum Vitae

1950 Born Warren Lee Tamahori on 17 June in Wellington, New Zealand to a Maori father and British mother.

1955 Attends Massey High School and, later, Tawa College. Leaves to work as a commercial artist and photographer.

1978 Boom operator for Television New Zealand and on Skin Deep, Goodbye Pork Pie and Bad Blood.

1986 Co-founds the commercial production company Flying Fish and makes his name with a series of high-profile television commercials. Appointed first assistant director on a number of film projects.

1994 Once Were Warriors gives him his big break as a filmmaker. Highest grossing NZ film to date.

1996 Directs his first American feature, Mulholland Falls, starring Nick Nolte, Melanie Griffith and John Malkovich.

1997 Works on The Edge, an adventure film with Sir Anthony Hopkins.

2000 Makes a brief return to television, directing an episode of The Sopranos.

2001 Directs Morgan Freeman in Along Came a Spider, to poor reviews.

2002 Directs Die Another Day, the 20th Bond film. Reviews are mixed but it is the highest grossing Bond film to date.

2005 State of the Union, starring Ice Cube, opens to poor financial and critical reception.